1. Field of the Invention
The present invention relates to a process for optical fiber drawing.
2. Description of the Related Art
In a conventional drawing process for producing an optical fiber, the optical fiber is produced by heating and melting a preform for the optical fiber in a drawing furnace and drawing the fiber from the preform at a certain rate by a winding up device. The optical fiber which has just left the furnace and is remaining intact, that is, a so-called "bare fiber", tends to be considerably damaged and influenced with moisture. Therefore, the bare fiber is usually coated with an ultraviolet curable resin or a thermosetting resin in a resin coating device comprising, for example, a die, the resin is consequently cured in a resin curing device, and then the fiber is wound as a coated optical fiber. A diameter of the bare fiber is measured by a measuring device before the coating steps, whereby conditions during the drawing are controlled so that the outer diameter of the fiber is to be a preselected one.
A position at which the diameter measuring device is disposed has not been thought to be critical, and the device is usually located immediately below the drawing furnace as shown in Japanese Patent Kokai Publication No. 295260/1986.
If there is anything to limit the position of the measuring device, it has been that the measuring device should not be directly subjected to a strong radiation light from a lower portion of the furnace to avoid being heated to a remarkably high temperature.
In addition, it is preferred to locate the measuring device near the furnace in order to shorten the time lag and to increase a control gain when fluctuation in the diameter of the optical fiber has to be suppressed by controlling a drawing rate depending on an output signal from the measuring device.
Thus, in the conventional production of the optical fiber, usually a distance between the outer diameter measuring device and the coating die is longer than that between the drawing furnace and the measuring device, or a forced cooling device is disposed between the measuring device and the coating die in order to achieve a better resin coating.
In the conventional drawing process for producing the optical fiber, the drawing rate of the optical fiber was in the order of 100 m/min. Recently, the drawing rate is remarkably increased and it is reported that, in an experimental scale, a rate of 1000 m/min. has been realized. However, when such a high drawing rate is employed in the conventional process in which the measuring device is located immediately below the furnace, it has been found that the outer diameter of the finished optical fiber is extremely smaller than the diameter which is measured with the measuring device. As the requirements for accuracy in optical fiber diameter increase and the minimization of fluctuation in that diameter become critical as the result of the connection between fibers becoming better, development of a process which improves the accuracy of the outer diameter of the optical fiber is highly desired.
For example, the accuracy of the diameter of a quartz base optical fiber is usually required to be in 125.mu.m.+-.1.mu.m. Taking account into an accuracy of the measuring device itself and the fluctuation in the diameter of the optical fiber during the production, a deviation of the measured diameter with the measuring device from a true diameter of the finished fiber should be not larger than 0.5% of the outer diameter of the finished fiber. Thus, it is desirable to develop a process which can achieve the deviation of 0.5% or less.